Phase-change VO2-based thermochromic smart windows

Thermochromic coatings hold promise in reducing building energy consumption by dynamically regulating the heat gain of windows, which are often regarded as less energy-efficient components, across different seasons. Vanadium dioxide (VO2) stands out as a versatile thermochromic material for smart windows owing to its reversible metal-to-insulator transition (MIT) alongside correlated structural and optical properties. In this review, we delve into recent advancements in the phase-change VO2-based thermochromic coatings for smart windows, spanning from the macroscopic crystal level to the microscopic structural level (including elemental doping and micro/nano-engineering), as well as advances in controllable fabrication. It is notable that hybridizing functional elements/materials (e.g., W, Mo/SiO2, TiN) with VO2 in delicate structural designs (e.g., core-shell, optical cavity) brings new degrees of freedom for controlling the thermochromic properties, including the MIT temperature, luminous transmittance, solar-energy modulation ability and building-relevant multi-functionality. Additionally, we provide an overview of alternative chromogenic materials that could potentially complement or surpass the intrinsic limitations of VO2. By examining the landscape of emerging materials, we aim to broaden the scope of possibilities for smart window technologies. We also offer insights into the current challenges and prospects of VO2-based thermochromic smart windows, presenting a roadmap for advancing this field towards enhanced energy efficiency and sustainable building design. In summary, this review innovatively categorizes doping strategies and corresponding effects of VO2, underscores their crucial NIR-energy modulation ability for smart windows, pioneers a theoretical analysis of inverse core-shell structures, prioritizes practical engineering strategies for solar modulation in VO2 films, and summarizes complementary chromogenic materials, thus ultimately advancing VO2-based smart window technologies with a fresh perspective.


Introduction
The escalating demand for energy consumption and the resultant emission of CO 2 have been intensifying the global-warming problem, emphasizing the pressing imperative for energy conservation.Reports indicate that residential buildings are responsible for nearly 30% to 40% of the world's primary energy consumption 1 .Hence, there is a significant imperative to enhance the energy efficiency of buildings.Among all building components, windows are frequently identified as less energy-efficient and requiring greater maintenance 2 , making smart windows an attractive research topic in optics, materials science, and building science.Windows play a crucial role in providing thermal, light, and acoustic comfort 3 , facilitating vision, air ventilation, photo-protection 4 , prevention of skin cancer 5 , and even influencing biopsychological effects 6 .The design of smart windows should take these aspects into consideration.Sunlight interacts with windows through transmission, reflection, or absorption, depending on their spectral properties in the ultraviolet (UV), visible, and near-infrared (NIR) regions.Consistently high transmittance in the visible spectrum is essential to meet lighting requirements.Indeed, a smart switch alternating between high and low transmittance in the UV and NIR spectral bands is desirable to minimize cooling loads during hot seasons and maximize heat gain during cold seasons.Thermochromic smart windows adjust radiation in invisible spectra according to surrounding temperatures, paving the way for highly efficient windows in the next generation of energy-conscious architectures.
Currently, commonly utilized thermochromic materials encompass vanadium dioxide, perovskite 7 , organic liquid crystal 8 , as well as mechano-thermo-chromic materials such as supersaturated salt hydrate crystal 9 .While supersaturated salt hydrates offer rapid and reversible phase transitions for smart window applications 9 , VO 2 stands out for its sharp and abrupt change in transparency at a specific temperature, providing precise and consistent performance without the need for mechanical triggers or complex electro-thermal systems.VO 2 is notable for the reversible temperature-dependent dielectric constants, exhibiting significant disparities between their metallic and insulating states.Rutile VO 2 (R) undergoes a reversible first-order MIT to monoclinic VO 2 (M) at a relatively moderate phase transition temperature (T c , 68 °C).The structure and optical properties can mutate before and after T c 10-13   , which makes VO 2 an ideal thermochromic material for smart windows over the years.
Here, we provide an overview of phase-change VO 2based thermochromic coatings for smart windows, drawing on the latest research and structured according to the logic depicted in Fig. 1.Firstly, the spectral transmittance modulation resulting from the MIT of VO 2 in crystalline structures is introduced, as illustrated in Fig. 1a.Further exploration is undertaken into the effects of the MIT on the thermodynamic, electrical, and optical properties of VO 2 , elucidated through band structures depicted in Fig. 1b.Secondly, elemental doping is introduced as Fig. 1c illustrates, considering the difference between the actual temperature and T c .Thirdly, we discuss the micro/nano-engineering (Fig. 1d) in the categories including hybridization, core-shell micro/ nanostructure, and multilayer films design, emphasizing on the structural effects for further improving the spectral modulation ability.Next, we review fabrication methods of VO 2 -based thermochromic films/coatings, with a specific focus on multi-functionalities, such as stability and emissivity, and the capability of controllable large-scale manufacturing, as depicted in Figs.1e-1f.Additionally, we briefly introduce and discuss other promising chromogenic materials that can potentially be integrated with VO 2 for smart windows to overcome their intrinsic limitations.Finally, outlooks and perspectives are addressed regarding current challenges and further advancements of VO 2 -based thermochromic smart windows.
This review distinguishes itself from previous works through the following unique contributions: 1. We provide a comprehensive categorization of VO 2 doping strategies, shedding light on the doping mechanism, and emphasizing the crucial NIRenergy modulation capability (ΔT NIR ) for smart window applications.2. We pioneer a theoretical analysis of inverse coreshell structures featuring VO 2 as the shell material, delving into their potential for enhancing solar modulation and offering novel insights for future research.3. We prioritize practical engineering strategies to boost the solar-energy modulation capabilities of VO 2 films, moving beyond mere descriptions of various VO 2 nanostructures.4. We offer a comprehensive summary of other chromogenic materials that can complement VO 2 to enhance smart window functionality, providing a broader perspective than previous studies.These elements inject a fresh perspective into VO 2based smart window technologies and are critical for further advancing the field.

Temperature-triggered phase change for solar modulation
The MIT in VO 2 is thermally induced, with concomitant alterations in the entropy and heat capacity of the material.In 1971, Dr. John B. Goodenough delineated molecular orbital diagrams grounded in crystal-field theory, as depicted in Fig. 1b 14 .Simply, there is a π bond and a π* anti-bond between the V 4+ and O 2-orbitals in VO 2 (R).Meanwhile, a d // nonbond is formed between nearby V 4+ orbitals along the crystallographic c axis, which partially overlaps with the unfilled π* band.Metallic characteristics arise from the overlap between π* and d // bonds, at which the Fermi level falls, leading to the VO 2 (R) with high electron mobility.As the temperature decreases, lattice distortion enhances the π overlap between the V 4+ and O 2-orbitals, elevating the energy level of the π* anti-bond.Concurrently, the interaction between V-V pairs induces the splitting of the d // bonds into d // -bonding and anti-bonding components, resulting in a band gap of ~0.7 eV between the π* and d // -bonding levels.
The presence of a band gap, resulting from the split between the π* and d//-bonding levels, gives rise to insulating properties in VO 2 (M).This band gap acts as a barrier to electron mobility, thereby leading to the manifestation of insulating behavior.While there may be variations between this theoretical proposal and experimental observations, it nonetheless offers valuable insights into the fundamental nature of the MIT in VO 2 .The fluctuation in electron mobility during the MIT enables switchable electrical conductivity, a characteristic that finds application in diverse fields such as tunable polarization converters 15,16 .In metallic VO 2 (R), the dielectric constant is considerably lower compared to insulating VO 2 (M).This discrepancy arises primarily because the presence of free electrons in VO 2 (R) allows them to screen the electric field effectively, diminishing the polarization of its lattice structure induced by external electric fields.As a result, the dielectric constant in metallic VO 2 (R) is notably reduced in comparison to its insulating counterpart.As depicted in Fig. 1a, VO 2 exhibits a monoclinic structure characterized by high transparency to visible and near-infrared (NIR) light below its phase transition temperature (T c ). Conversely, upon transitioning to the rutile structure (R) above Tc, the material maintains nearly consistent visible light transmittance but experiences a significant reduction in NIR transmittance [17][18][19][20] .To evaluate the optical performance of a VO 2 -based thermochromic smart window, the luminous transmittance (T lum , 380-780 nm), the solarenergy modulation ability (ΔT sol , 300-2500 nm) and the NIR-energy modulation ability (ΔT NIR , 780-2500 nm) are calculated as follow: where Φ lum (λ) is the standard luminous efficiency function of human photosensitive vision [21][22][23] , Φ sol/NIR (λ) is the solar/NIR irradiance spectrum at air mass 1.5 (corresponding to the sun standing 37°above the horizon) 24 , T(λ) and T lum/sol/NIR denote the spectral transmittance at wavelength λ and luminous/solar/NIR transmittance, respectively., American Chemical Society It's noteworthy that T lum differs from visible solar transmittance (T vis ) 25 , which considers the relative spectral distribution of illuminant D65 26 .For VO 2 -based smart windows, ΔT NIR is approximately double ΔT sol , as noticeable spectral transmittance changes occur in NIR bands (~52% of total solar irradiance) before and after MIT.Both high T lum and ΔT sol/NIR are crucial: the former signifies better lighting savings, while the latter determines the energy-saving performance of VO 2 -based smart windows.However, constrained by the intrinsic MIT, VO 2 -based smart windows face obstacles such as high transition temperature (68 °C), low luminous transmittance (T lum < 60%), and weak solar-energy modulation (ΔT sol < 15%).Extensive efforts have been dedicated to addressing these challenges and advancing VO 2 -based smart windows for practical applications.

Elemental doping for tailoring the phasetransition temperature
As mentioned above, the primary challenge faced by VO 2 -based smart windows in practical applications is the significantly higher MIT compared to the normal operating temperature of conventional windows.To address this issue, researchers commonly employ elemental doping as the primary strategy to modulate T c and optical properties of VO 2 .The effects of some doping elements are summarized in Table 1.
The fundamental role of dopants is to lower the energy barrier, thereby reducing the MIT temperature, as the electronic phase transition in VO 2 nearly coincides with the structural phase transition 27,28 .Correspondingly, the selection of the elemental doping is commonly based on two key factors: (1) Increasing the carrier concentration to accelerate the electronic phase transition: Dopants are selected to either donate additional electrons (n-type doping) or create spaces for electrons through the formation of vacancies (p-type doping).This tactic increases the mobile charge carrier density within the VO 2 lattice, thereby amplifying the likelihood of electronic excitations that can precipitate the phase transition.By bolstering the number of available charge carriers, the dopants accelerate the electronic phase transition.This acceleration is due to the heightened probability of electronic interactions that can drive the system across the phase boundary, effectively diminishing the energy threshold for the MIT; (2) Introducing distortion into the atomic structure to assist the structural phase transition: The second prong of the doping strategy involves the deliberate introduction of atomic-scale distortions.These distortions arise from the mismatch between the dopants and the host VO 2 lattice in terms of size or valence, leading to localized strain.Such strain can perturb the equilibrium of interatomic forces within the crystal, thus lowering the energy required for the structural reconfiguration that accompanies the MIT.The dopants act as a catalyst for structural changes by stabilizing the high-temperature metallic phase, which is characterized by a distinct arrangement of V-V dimers.This stabilization aids in the rapid reorganization of the vanadium and oxygen atoms, facilitating the transition from the insulating to the metallic state.
Regarding the increase in carrier concentration, researchers have pursued two primary doping strategies: a) insert smaller-sized doping atoms (Fig. 1c, left) such as H 29,30 , Li 31,32 , Na 31,33 , and B 34,35 into the interstitial sites of VO 2 ; b) substitute the V sites with high-valance elements (Fig. 1c, mid) such as W 36,37 , Mo 38 , Nb 39 .These strategies aim to enhance the concentration of mobile charge carriers within VO 2 crystals, thus lowering T c .For instance, when smaller dopant atoms are inserted into the interstitial sites of VO 2 lattice, they introduce additional electrons, thereby increasing the carrier concentration within VO 2 crystals.Similarly, by substituting vanadium sites with high-valence elements, these elements can inject partial electrons into the valence band of VO 2 .For example, by substituting vanadium atoms in VO 2 , tungsten atoms can inject electrons into the valence band of VO 2 , effectively lowering T c , typically by 20-26 K/at%.
On the other hand, to address structural distortion, researchers have also employed methods such as adjusting the V-V distances in VO 2 or modifying lattice parameters by a dopant (Sb 40 , Be 41 , etc.).Through these measures, researchers can effectively modulate the crystal structure of VO 2 , thereby affecting its transition temperature T c .For example, by adjusting the distances between vanadium atoms in VO 2 crystals or by modifying lattice parameters, the structure of VO 2 crystals can be altered to facilitate phase transition.These adjustments and distortions in structure can effectively reduce the transition temperature T c of VO 2 , making it more suitable for applications such as smart windows.
In conclusion, elemental doping serves as an effective method for modulating the critical transition temperature of VO 2 -based smart windows, with selection based on precise control of carrier concentration and structural distortion.This strategy provides crucial technical support for enhancing the performance and applicability of VO 2 -based smart windows in practical applications.In future research, a deeper understanding of the doping mechanism and various influencing factors in the doping process will be essential.
While doping strategies have been successful in modulating the transition temperature (T c ), the environmental stability and long-term performance impact of doped elements must be considered.Greater emphasis should be placed on the selection of dopants and the optimization of doping levels in future research to ensure more stable and efficient thermochromic smart windows.

Micro/nano-engineering for Improving the solarenergy modulation ability
In addition to lowering T c , elevating T lum and ΔT sol is also important for smart window applications 42 .Micro/ nano-engineering strategies through either micro-or macro-structural/materials modification have been proposed and investigated widely 43 .According to the modification principles, the reported micro/nano-engineering strategies can be classified into three categories: 1) hybridization that incorporates other materials/structures with individual VO 2 materials, 2) core-shell micro/nanostructures that modulate the Mie/plasmonic resonant responses of spherical VO 2 nanoparticles (NPs) as either core or shell, and 3) multilayer films that integrate other flat layers for enhanced optical resonances as well as multifunctionality.

VO 2 -based hybridized thin films
In efforts to improve the thermochromic properties of VO 2 -based thin films, researchers often encounter a trade-off between ΔT sol and T lum when using thick layers of continuous VO 2 film.To address this challenge, hybridization with other inorganic and/or organic materials has emerged as a viable strategy to optimize both T lum and ΔT sol simultaneously.This hybridization approach may affect the MIT and hysteresis-loop width of VO 2 by inducing strains, while also modulating optical constants such as T lum and color 44 .Furthermore, the introduction of functional materials like TiO 2 45 and ZrO 2 46 can add multifunctionality to the composite films, offering benefits such as self-cleaning and mechanical reinforcement.Researchers have explored various strategies to improve the optical performance of VO 2 -based thin films.Li et al. theoretically investigated the optical performance of VO 2 NPs matrix composites and yielded much higher T lum and ΔT sol than pure VO 2 thin films by calculations based on effective medium theory 47 .Liang et al. further introduced SiO 2 microparticles in the W-doped VO 2 /PVP composites for T lum improvement.The single layer of randomly dispersed SiO 2 microspheres in the film provided optical pathways for sunlight, which offset the decrease of T lum caused by the strong absorption of visible light by high volume concentration of W-VO 2 NPs (Fig. 2a).Experimental results revealed that W-doped VO 2 /SiO 2 /PVP composite films enable high T lum of 65% without sacrificing too much ΔT sol (12.6%) 48, which also proved the validity of the structure in the previous study by Li et al. 47 .Hao et al. proposed an alternative strategy that utilize plasmonic response to balance T lum and ΔT sol .A plasmonic array of TiN NPs was fabricated underneath the VO 2 thin film as Fig. 2b exhibits and endowed ΔT sol of 10.8% and T lum of 51%, respectively 49 .TiN NPs can efficiently absorb NIR radiation to provide local heating and accelerate the MIT in VO 2 .Additionally, the VO 2 film can also be patterned for performance improvement.Cao et al. synthesized   34,35 Substitutional site "-" means data unavailable periodical and spherical cavity-structured VO 2 /SiO 2 composite films through a polystyrene template and annealing process as illustrated in Fig. 2c.Under the photonics crystal structure of the patterned VO 2 composite film, balanced thermochromic properties can be obtained as ΔT sol of 8.4% and T lum of 55.6% 50 .Inorganic VO 2 could also be integrated with organic materials to enhance the thermochromic properties.For instance, He et al. presented a dynamically regulated system based on W-VO 2 /PAM-PNIPAM hydrogel films, which was developed into smart windows (ΔT sol 46.3% and T lum 72%) 51 .As illustrated in Fig. 2d, the PVP-modified monoclinic W-VO 2 was dispersed in the PNIPAM microgel, where the PAM hydrogel with high T lum serves as a skeleton, holding and keeping water molecules, PNIPAM microgel and surface-modified W-VO 2 inside.Upon heating, the intramolecular and intermolecular hydrophobicity of PNIPAM is enhanced, leading to the discharge of water molecules from the chemical framework of PNIPAM and generating phase separation interfaces.These interfaces can strongly scatter the incident solar irradiance when the W-VO 2 undergoes the MIT and hence dramatically enhance the NIR light reflectivity simultaneously.
VO 2 -based core-shell micro/nano-structures In addition to hybridization methods, core-shell approaches have emerged as an efficacious strategy for enhancing the thermochromic properties and functionalities of VO 2 -based films 52 .These core-shell nanostructures can be broadly categorized into bilayer and trilayer configurations based on the number of layers, wherein VO 2 is encapsulated within dielectric shells such as SiO 2 , TiO 2 , SnO 2 , ZnO, Al 2 O 3

53
, or metallic shells like Au and Ag.For instance, Pi et al. prepared thermochromic and hydrophobic core-shell VO 2 @SiO 2 (VSQ) NPs with tetraethoxysilane (TEOS) and dimethyloctadecyl [3-(trimethoxysilyl) propyl] ammonium chloride (DMOAP) as depicted in Fig. 3a 54 .The ΔT sol reaches up to 15.4% while the T lum remains as high as 51.5% owing to localized surface plasmon resonance (LSPR) 55 .Furthermore, the SiO 2 shell efficiently shields VO 2 from oxidation, enhancing its durability, while DMOAP modification imparts additional functionalities including anti-agglomeration, surface hydrophobicity, and self-cleaning, as illustrated in Fig. 3b 54 .Lu et al. synthesized VO 2 @SiO 2 @Au trilayer core-shell nanoparticles to harness LSPR and enhance optical tunability, as depicted in Fig. 3c 56 .By modulating the material composition and particle sizes of the outer metallic shell, the resonance peak could be tuned across a wide spectral range, offering customizable colorful appearances for the films, as shown in Fig. 3d.Since VO 2 can transition between dielectric and metallic states, it can be employed as either the inner or outer layer of core-shell structures.Yao et al. fabricated SiO 2 @TiO 2 @VO 2 inverted trilayer core-shell NPs via From left to right: a VO 2 @SiO 2 @Ag NPs-based film (V), a plain VO 2 film (I), VO 2 @SiO 2 @Au films grew in different content of HAuCl 4 (II-IV) 56 .e Synthesis procedures of SiO 2 @TiO 2 @VO 2 inverted trilayer core-shell NPs 57 .f SEM micrographs of (f1) a SiO 2 hollow nanosphere-based coating, (f2) a SiO 2 @TiO 2 coating, and (f3) a SiO 2 @TiO 2 @VO 2 coating, respectively 57 and SiO 2 @TiO 2 @VO 2 coatings at 20 and 100 °C57 , and c VSQ coatings treated at 60 °C and 90% RH for different periods 54 .d Absorption spectra of different solutions 56 .e, f ΔT sol of VO 2 core-shell NPs-based films, as a function of e shell refractive index and f relative shell thickness 53 .g, h ΔT sol of VO 2 inverted core-shell NPs-based films, as a function of g core refractive index and h relative shell thickness, the performance of the plain VO 2 film was shown by the dashed lines.Figures reproduced with permission from: a, Chen et al. 58 , American Chemical Society controlled interfacial engineering, as depicted in Fig. 3e.The T lum of the resultant coating reached a remarkable 74%.Additionally, as Fig. 3f illustrates, since TiO 2 was partially exposed rather than completely covered by VO 2 , the coating facilitated the integration of thermochromism from the outer VO 2 layer, photocatalytic self-cleaning capability from the middle TiO 2 layer, and antireflective properties from the internal SiO 2 hollow nanospheres 57 .
The spectral transmittances of typical bilayer and trilayer VO 2 -based core-shell structures are shown in Fig. 4a, b, demonstrating a notable enhancement in T lum .Additionally, Fig. 4c, d illustrates the durability and coloration of these core-shell configurations, providing detailed insights into their multifunctional capabilities.Specifically, Fig. 4c showcases the VSQ coating's ability to enhance the stability and longevity of VO 2 by providing a protective SiO 2 shell, preventing oxidation, incorporating hydrophobic modification for self-cleaning, and maintaining structural integrity through robust adhesion.This innovative approach collectively improves environmental and mechanical durability, addressing crucial challenges faced by VO 2 -based thermochromic coatings 55 .Moreover, in Fig. 4d, the tunable SPR properties of Au nanoparticles are highlighted, demonstrating their potential to modify the color of solutions and, by extension, the color of VO 2 -based thermochromic smart films 57 .Lu et al.'s work illustrates how controlling the size of the Au nanoparticles enables the tailoring of film color from brick red to purple and blue.This advancement represents a significant stride in the development of smart window technologies, offering versatile solutions for enhancing the aesthetic appeal and functionality of VO 2based coatings.The thermochromic behavior of VO 2based core-shell structures is intricately linked to the properties of the core/shell materials and their respective sizes, which govern the dielectric environment surrounding VO 2 54 .To systematically investigate the optical performance of the thermochromic films, Xie et al.
performed theoretical calculations based on the effective medium theory coupled with the transfer matrix method.Their findings, showcased in Fig. 4e-f, suggest that for films composed of VO 2 core-shell nanoparticles, selecting a low absorption shell material (e.g., ZnO or Cr 2 O 3 ) with a refractive index (RI) ranging from 1.6 to 2.3, and maintaining a relative shell thickness between 0.1 and 0.3, is crucial for preserving a high sol-gel transition temperature difference (ΔT sol ) 53 .Here, the relative shell thickness (α) is defined as the ratio of the shell thickness to the core radius (α = t shell /R core ).In Fig. 4e, dashed lines represent the solar modulation capability of VO 2 nanoparticle, while solid orange lines represent VO 2 @shell structures with varying shell materials.Analysis indicates that VO 2 -based core-shell structures outperform VO 2 nanoparticles when the shell refractive index falls within 1.6 to 2.3, yielding performance enhancement.Similarly, in Fig. 4f, for VO 2 @ZnO or VO 2 @Cr 2 O 3 structures, superior performance of VO 2 -based core-shell structures is observed with shell thickness between 0.1 and 0.3, demonstrating performance improvement.Table 2 has been included, detailing the distinct performance and characteristics of VO 2 -based core-shell micro/nanostructures with various dielectric and metallic shell materials.
However, the scarcity of investigations into the optical characteristics of inverted core-shell structures, where VO 2 serves as the shell material, underscores a critical gap in our current understanding.To address this gap, we conducted theoretical analyses utilizing the Mie theory coupled with the Monte Carlo method.Our research specifically targeted inverted VO 2 core-shell nanoparticles incorporating core materials such as SiO 2 , ZnO, Cr 2 O 3 , and TiO 2 .
Upon scrutinizing Fig. 4e, g, a notable trend emerged: the inverted structures consistently displayed superior solar modulation capability across all material combinations compared to their conventional counterparts.Furthermore, it was observed that the ΔT sol of these films

Multilayer films design with photonic cavity
In general, there are mainly two forms of thermochromic coatings based on VO 2 : flexible foils and multilayered films.Flexible foils involve dispersing VO 2 -based nanoparticles in a polymer host matrix such as PU, PVP, and PMMA, and then combining them with a flexible substrate.On the other hand, multilayered films are typically fabricated on glass using direct deposition methods such as physical vapor deposition (PVD) and chemical vapor deposition (CVD).Since the absorption of VO 2 is mainly in the 250-700 nm wavelength range, the T lum of single-layer VO 2 films is generally restricted.Additionally, due to its transmittance switches primarily in the NIR region, which accounts for about 43% of solar energy in the solar spectrum, leading to limited solar modulation (ΔT sol ), often remaining below 10%.
Multilayer films design is an effective solution towards this perplexity.To address these limitations, multilayer film designs have emerged as effective solutions.These films not only allow for the design of solar transmittance but also enable the tailoring of NIR emissivity 58 .Moreover, the integration of different layers makes it easier to achieve multifunctionality compared to single-layer films.
Antireflection (AR) is a noteworthy function that has been demonstrated as a viable method to enhance the low luminous transmittance (T lum ) of VO 2 -based films without compromising their thermochromic properties.Additionally, neat VO 2 is prone to oxidation to V 2 O 5 in air and exhibits poor acid resistance, leading to environmental instability that limits its practical application as a thermochromic coating in smart windows 59 .Incorporating VO 2 films with other layers can introduce practical functions such as antioxidizability 60 , hydrophobicity 61 , and photocatalysis 62 .For instance, Zheng et al. designed a TiO 2 /VO 2 /TiO 2 multilayer film (with ΔT sol of 10.2% and T lum of 30.1%), which exhibited at least three functions: antifogging/self-cleaning, thermochromic, and antireflective properties.These properties were respectively attributed to the top TiO 2 (A), the middle VO 2 (M), and the bottom TiO 2 (R) layers, as illustrated in Fig. 5a 63 .However, as for the antireflection coatings (ARCs), most  66 , American Chemical Society researchers focused on increasing luminous transmittance (T lum ) at the expense of sacrificing solar modulation (ΔT sol ).This trade-off is a significant challenge as ΔT sol is a determinant factor for energy-saving applications and can hinder the translation of these technologies from the laboratory to the market.To address this challenge, variable ARC/VO 2 multilayer film designs, as illustrated in Fig. 5b and c Additionally, it can actively control the amount of solvent by detecting the temperature difference and humidity between the insulating glass, thereby adjusting the thickness of the ARC 65 .Thermal emissivity (ε T ) holds equal importance alongside ΔT sol and T lum for VO 2 -based smart windows.It has been leveraged in numerous thermal management and energy-saving applications [66][67][68][69] and has recently garnered attention in the context of smart windows, particularly in combination with the concept of radiative cooling [70][71][72] .Radiative cooling harnesses the infrared atmospheric transparent window to dissipate heat and effectively cools terrestrial objects through thermal radiation [73][74][75] .
In comparison to traditional smart windows, which primarily focus on modulating incoming solar energy, high thermal emissivity (ε T ) for radiative cooling offers an additional channel for dissipating excessive thermal energy generated by solar heating.According to Kirchhoff's thermal law, the absorptivity of an object is equal to its emissivity in thermal equilibrium conditions.And the transmittance of glasses can be ignored when the wavelength is longer than 4.5 μm.Commonly, ε T is calculated by weighting the film reflectance with the black-body emission spectrum from 4.5 to 25 µm as follow: where G T (λ) is the normalized relative spectral distribution of black-body radiation at temperature T (T is chosen to be 20 °C according to CNS GB/T 1895.2-2002).E(λ) refers to the spectral emittance, i.e., the fraction of the black-body radiation.R(λ) refers to the reflectance in the region (4.5-25 µm).
A high value of ε T indicates an intensive energy exchange between the window surface and its ambience through thermal radiation and absorption 76 .Wang et al. developed a passive radiative cooling regulating thermochromic (RCRT) smart window (ΔT sol 17% and T lum 32.2%) with tunable long-wave infrared (LWIR) emissivity (ε LWIR ) based on a W-doped VO 2 -PMMA/spacer/low-E stack using spin coating, as illustrated in Fig. 5d.This configuration formed a Fabry-Pérot resonator, enabling the promotion of radiative cooling during warm weather (high ε LWIR ) or suppress it during cold weather (low ε LWIR ) 70 .
To further elucidate the energy-saving benefits of VO 2based smart windows, we calculated the annual total energy savings for model buildings equipped with multilayer VO 2based smart windows (ΔT sol = 10%, Δε = 0.5). Figure 5e illustrates the total energy saved annually in various cities across China.From the energy-saving map, it is evident that cities in southern regions exhibit more pronounced energysaving effects compared to those in northern regions, primarily due to the hotter climate.For instance, the average annual energy savings in Hong Kong amount to 80.16 kWh/ m 2 , whereas in Jilin, it is only 45.12 kWh/m 2 .This analysis underscores that the installation of VO 2 -based smart windows in buildings leads to reduced energy consumption, yielding significant energy-saving benefits.
It is worth noting that achieving dynamic modulation of emissivity can be challenging when incorporating low-E coatings (such as ITO) due to their strong NIR blocking properties, which can lead to relatively low values of ΔT sol .The simultaneous modulation of the spectral performance in both the NIR and long-wave infrared (LWIR) regions remains a question to be addressed in future research.
Additionally, Tang et al. introduced a W x V 1-x O 2 -based temperature-adaptive radiative coating (TARC), as shown in Fig. 5e, to achieve strong sky-window radiative cooling at high temperatures and solar heating or warm-keeping at low temperatures.This was made possible by amplified radiative absorption facilitated by the designed Fabry-Pérot resonance with adjacent W x V 1-x O 2 blocks, as well as the bottom Ag layer within the ¼-wavelength cavity 71 .Furthermore, Liu et al. presented a VO 2 -PDMS-driven intelligent radiative cooling coating with both automatic cooling switching and continuous adjustment of thermal comfort, as illustrated in Fig. 5f 77 .The polydimethylsiloxane (PDMS) substrate could be stretched to modulate the radiative cooling power according to our desired cooling temperature.Adjusting the VO 2 -PDMS coatings through stretching modifies the filling ratio, consequently influencing the dielectric characteristics of the stacked nano-grating structure.This alteration impacts surface waves at the interfaces, resulting in modifications to the spectral performance of the coatings.
Micro/nano-engineering, offering new degrees of freedom in the design and manufacturing of VO 2 -based thermochromic materials, also presents challenges regarding manufacturing complexity and cost control.It is suggested that future efforts should be directed towards streamlining the manufacturing process and reducing costs, without compromising or enhancing material performance.
To develop high-performance and cost-effective VO 2 -based smart windows, researchers must gain a deep understanding and precise control of the doping mechanisms, utilize micro/ nano-engineering techniques to optimize optical properties, and balance T c with optical characteristics.By employing precise doping strategies and micro/nano-engineering techniques, T lum and ΔT sol can be optimized.Additionally, simplifying the manufacturing process to reduce costs is essential to ensure the product's performance and economic viability in practical applications.

Preparation of VO 2 nanoparticles
High-quality VO 2 NPs with narrow distribution and uniform morphology are undoubtedly conducive to fabricate ideal thermochromic films.Recently, several methods have been reported to prepare definitive VO 2 particles, as shown in Table 3.For instance, solid-phase reaction methods offer low-cost and large-scale production but may involve toxic reactants and impurities, while gas-phase reaction methods provide morphological control but are often constrained by conditional reactions and equipment complexity.
CVD and PVD methods are typically constrained by expensive and intricate equipment, often resulting in low yield.The hydrothermal method usually necessitates precise control over temperature and atmosphere, while ball-milling and thermal reduction methods are prone to introducing impurities.In contrast, the pyrolysis process offers advantages such as mild reaction conditions, controllable product composition, and minimal impurity introduction, making it conducive to large-scale production.Recent advancements, such as the combination of solvent-thermal and pyrolysis processes, have led to the synthesis of purer VO 2 powder with high crystallinity.Hua et al. employed a combination of solvent-thermal and pyrolysis processes to synthesize purer VO 2 powder under mild air conditions.Through manual grinding and etching processes, they obtained innovative VO 2 (M) nanoparticles with high crystallinity 55 .The equations for the aforementioned processes are as follows: The preparation of VO 2 nanoparticles is a critical step in the development of VO 2 -based thermochromic smart windows.While various methods have been explored, each with its own set of advantages and limitations, the pyrolysis process stands out for its mild reaction conditions and minimal impurity introduction, which are particularly beneficial for large-scale production.

Fabrication of VO 2 -based thermochromic films
Thus far, a multitude of methods have been devised to produce high-quality VO 2 films with outstanding thermochromic properties [78][79][80] .These methods can be broadly categorized into chemical and physical techniques.Among them, CVD stands out as a commonly employed fabrication method for VO 2 thin films, owing to its merits of high deposition rate, uniform structure, excellent adhesion, and scalability.In the CVD process, a precursor containing metal ions is dissolved in a solvent and then transported by an inert carrier gas to the deposition reactor, where the chemical reaction occurs, as illustrated in Fig. 6a.For instance, electric-field assisted chemical vapor deposition (EACVD) 78 and low temperature chemical vapor deposition (LTCVD) 80 have been developed to enhance thermochromic properties and facilitate low-cost production.
Guo et al. prepared thermochromic VO 2 thin films (ΔT sol 9.7% and T lum 52.3%) by LTCVD at a lower T c .This reduction in Tc was attributed to the combined effects of strain and the presence of oxygen vacancies in the films during annealing.However, there has been limited emphasis on comparatively large-scale production, and the precursors commonly used as vanadium sources in the CVD process are typically toxic and environmentally unfriendly, such as VCl 4   81   , VO(acac) 2 82 , VO(OC 3 H 7 ) 3 83 , etc. PVD is an extensively researched technique for depositing high-quality VO 2 thin films, particularly for applications like smart windows, due to its ability to produce highly homogeneous products, good repeatability, and potential for large-scale production.Examples include magnetron sputtering (MS) 63,84 and pulsed laser deposition (PLD) 85 , etc.
As illustrated in Fig. 6b, MS is a vacuum process used to deposit thin films on substrates by applying a high voltage across a low-pressure gas (usually argon) to create a "plasma".Energized plasma ions strike the target and cause atoms from that target to be ejected with enough energy to travel to and bond with the substrate.While PVD methods offer superior quality and crystallinity in the fabrication of VO 2 thin films compared to other techniques, the complexity and expense of the equipment, along with high maintenance costs, severely limit their practical widespread application in smart windows 86 .Furthermore, persisting issues include poor antioxidizability and weatherability 87 .Hence, there is a demand for facile, low-cost, eco-friendly, scalable, and high-quality fabrication methods for VO 2 -based thin films.
Electrospinning is a technique wherein a high electric field is directly applied to obtain fibers with diameters ranging from nanometers to micrometers 88 .This process typically comprises four components: a high-voltage power supply, a syringe pump, a spinneret, and a collector, as illustrated in Fig. 6c 89 .It is commonly employed in the fabrication of organic-inorganic film materials 90 .
In comparison with existing methods, the electrospinning technique is straightforward, mature, versatile, economical, and efficient for preparing continuous nanofibers with high porosity.These fibers have been proven to possess exceptional optical properties suitable for radiative-cooling applications 91 .Li et al. utilized electrospinning to prepare W-doped VO 2 /PVP composite fiber mats and identified two modes of the multiple scattering-absorbing process when light was absorbed by the fiber mats 92 .Due to the constitution of the fibers in the electrospun film, internal scattering is one of the reasons of low T lum for electrospun thermochromic films.Therefore, proper T lum -enhanced techniques are necessary.Lu et al. firstly fabricated PMMA-VO 2 transparent composite smart film (ΔT sol 6.9% and T lum 21%) with antioxidizability and super-hydrophobicity by electrospinning method, attributed to the gathered VO 2 embedded in the fiber and aligned along the fiber axis.Subsequent heat treatment process significantly enhanced its T lum 87 .Spin coating is a solution-based technique employed to deposit uniform thin films onto flat substrates.Typically, a small amount of coating material is applied to the center of the substrate, which is then rotated at high speed to spread the coating material through centrifugal force, as illustrated in Fig. 6d.Zhao et al. synthesized W-VO 2 thermochromic films (ΔT sol 8.8% and T lum 41.5%) on an amorphous glass substrate using a simple spin coating method, which is economical, straightforward, and practical.However, high-speed spinning becomes challenging as the substrate size increases, making film thinning difficult.Additionally, the material efficiency of spin coating is relatively low 93 , which limits its application for largescale production.
In film casting, a solution (referred to as "dope") of a polymer in a solvent is prepared and then applied to a moving substrate belt with a smooth surface 94 .The wet film levels on the belt before entering the drying zone, where the solvent is completely removed to form a solid layer.Subsequently, the dry film is peeled from the belt, which then cycles back to the coating station, as depicted in Fig. 6e.Gao et al. were the first to prepare transparent and flexible VSQ composite foils using an all-solution casting process.These foils exhibited a UV-shielding property and excellent thermochromic properties (ΔT sol 15.5% and T lum 41%) 95 .In addition to wet coating with a solution, hot-melt casting is another method utilized for film deposition.A formulation is cast into a film from either hot melt or solution without solvent.The casting process offers several advantages over other methods, including simplicity of mold, low cost, absence of residual stresses, and suitability for lowproduction applications.It provides superior optical characteristics with a high degree of flatness compared to flatsheet extrusion.However, casted parts may exhibit significant shrinkage post-solidification.
3D printing, or additive manufacturing, is a process for creating three-dimensional solid objects from a digital file.It encompasses various techniques, such as direct ink writing (DIW), stereolithography (SLA), selective laser melting (SLM)/selective laser sintering (SLS), and inject printing, among others.For example, one method involves selectively depositing droplets of printing materials to form the desired structure, utilizing an X-Y-Z three-axis motion platform, spray heads, and auxiliary curing devices 96,97 , as depicted in Fig. 6f.During the printing process, the fluid is ejected from the nozzle.With the movement of the platform, the ejected droplets are accurately deposited on the printing platform, and then solidified and formed at room temperature and atmospheric pressure.This method is low-cost, fast, efficient, flexible and suitable for the roll-to-roll process of complex structures, especially for materials that are difficult to form continuous wires.
.b Physical vapor deposition (PVD) 175 .c Electrospinning 89 .d Spin coating 176 .e Film casting 177 .f 3D printing 178 straightforward large-scale fabrication of uniform VO 2 thermochromic films using the inkjet printing technique sol : 32.4%).Large area VO 2 films (560 cm 2 ) could be obtained with excellent uniformity, with the difference in NIR transmittance of regions on the film being within 0.3% 98 .
The fabrication of VO 2 -based thermochromic films is a multifaceted process with a variety of techniques offering different benefits.CVD and PVD are recognized for their high-quality film production, yet their scalability and environmental impact present challenges.Electrospinning and spin coating provide more accessible and costeffective alternatives, with the potential for large-scale production.
Despite the of existing fabrication methods at the laboratory scale, challenges remain in scaling up production and ensuring environmental adaptability.Future research is recommended to focus on the development of scalable, environmentally friendly, and costeffective fabrication techniques.Additionally, it should explore new materials and structural designs to further enhance the performance and stability of VO 2 -based thermochromic films.

Other chromogenic materials
Despite significant efforts to enhance the optical responses of VO 2 -based thermochromic films, the overall T lum and ΔT sol remain constrained due to the intrinsic absorption of VO 2 .In the following section, we delve into a variety of alternative chromogenic materials that exhibit significant optical property changes, which are extensively utilized to modulate solar radiation.The potential for synergistic integration of these materials with VO 2 will be explored, with the aim of further refining spectral transmittance.Prominent among these are electro-, gaso-, mechano-, photochromic materials, each of which is activated by specific stimuli such as an electric field, gas exchange, mechanical stress, and light exposure.These smart chromogenic technologies each bring a unique set of advantages and challenges, and they are the focus of continuous innovation to improve their functionality and efficiency.The characteristics of various chromogenic materials are summarized in Table 4.
Electrochromic (EC) materials could reversibly change the optical properties under the influence of electric field, including inorganic metal oxides (TiO 2 99 , etc.), organic polymers (propylenedioxythiophene 100 , etc.), inorganicorganic hybrids: coordination complex 101 , prussian blue 102 , liquid crystal (Fig. 7a) 8 , etc. EC applications are composed of EC material, electrolyte, ion storage layer, etc. and sandwiched by two transparent conductive electrodes 103 .The advantages of electrochromic (EC) materials include easy operation through a control system and integration into the Internet of Things (IoT).
However, challenges such as poor UV durability (especially for polymer-based EC materials), coloration efficiency, cycling stability, and indoor-use restrictions remain to be addressed in the future.Sang et al. proposed a smart window (ΔT sol : 32.4% and T lum : 0.4-40%) with a dual-band modulation function based on a simple threelayer structure of liquid crystal, VO 2 and Al-doped ZnO, which can modulate the visible light actively by an external electric field and NIR light passively by temperature 104 .
Gasochromic (GC) materials are a type of thermochromics displaying a color change responding to a chemical change or reaction 105,106 .WO 3 is a well-known example of GC materials.In a double-glazed structure, the internal surfaces coated with a WO3 film exhibit reversible light transmittance switching when alternately exposed to diluted hydrogen and oxygen gases, as illustrated in Fig. 7b.One advantage of GC materials, such as WO 3 , is that the depth and switching speed of color change can be controlled by adjusting the film thickness and the concentration of hydrogen and oxygen gases.Due to their straightforward and cost-effective layer configuration, as well as high light transmittance, WO 3 -based GC devices have garnered significant attention since the beginning of the 21st century 107,108 .However, a major limitation of these devices is the requirement for a system to provide hydrogen and oxygen gases.
Mechanochromic (MC) materials respond to mechanical stimuli which can deform and reconstruct the surface morphologies or internal structures with a change of their optical properties 109 , like polyvinyl alcohol (PVA)/ PDMS 110 , diarylbibenzofuranone (DABBF) (Fig. 7c), etc.The MC applications are facile, eco-friendly, and responsive.The main obstacles to their commercialization include the significant applied biaxial strain and area expansion resulting from stretching, as well as the improvement of the fatigue life of cracks under high strength mechanical loading cycles without incurring catastrophic structural damage 111,112 .Ke et al. presented a bio-inspired thermo-mechanochromic dual-mode VO 2 / PVA/PDMS window (ΔT sol : 11.5% and T lum : 17-60%), to respectively control the scattering for privacy and the absorbance for energy-efficiency, which are attributed to the controllable broadband diffraction via the dynamic wrinkles and VO 2 NPs based on LSPR 113 .
Photochromic (PC) materials undergo reversible transformation when exposed to specific wavelengths of light, which could be roughly divided into organic and inorganic PC materials.Common examples include fulgide 114 , diarylethene 115 , spiropyran 116 (as depicted in Fig. 7d), TiO 2 117 , etc. Compared to inorganic counterparts, organic PC materials possess great advantage in structural diversity, photochromic reversibility, and processibility 118,119 .However, simply removing the UV is not sufficient to restore most organic PC materials to initial color (except for naphthopyran) 119,120 , and more basic research is needed to enrich the library of photochromism.Sang et al. fabricated a novel VO 2 /spiropyran composite film (ΔT sol 23.6% and T lum 31.55-48.6%),which could simultaneously modulate UV, visible, and NIR light with obvious color change from yellow to pink reversibly 121 .
Thermochromic materials enable a change in optical properties based on the temperature 122 , which is suitable for passive light modulation.In addition to VO 2 48 , perovskites (Fig. 7e) 123 and hydrogels 51 are also very representative.have their own spectral characteristics towards application scenarios in demand.
The exploration of alternative chromogenic materials offers a promising avenue for addressing the limitations of VO 2 -based thermochromic coatings.These aforementioned materials demonstrate significant changes in optical properties and are widely utilized in solar radiation modulation.Integrating these materials with VO 2 has the potential to further enhance functionality in smart windows.Each of these chromogenic technologies presents unique advantages and challenges, spurring continuous innovation to enhance their efficiency and performance.Through synergistic integration, these materials can enable smart windows to achieve improved optical responses and multifunctionality, thus facilitating their broader adoption in energy-efficient building applications.

Conclusion and outlook
Here, we present a comprehensive review on the VO 2based thermochromic coatings for smart windows.With the inspiring MIT transition, VO 2 serves as a promising material for spectral transmittance modulation.Given the limitations of pristine VO 2 , such as its high transition temperature, low luminous transmittance, and weak solar-energy modulation ability, a variety of research approaches have been proposed, spanning from the crystal level to the structural level, including elemental doping and micro-nano structural engineering.Based on the latest researches, we discuss the principles and results of micro/nano-engineering in detail by categories including hybridization, core-shell and multilayer film design.Moreover, we summarize the fabrication methods of VO 2 NPs and VO 2 -based thermochromic films.We also provide discussions on other chromogenic materials, which can be integrated for thermochromic smart windows as an alternative strategy to surpass the intrinsic limitation of VO 2 .Besides the above conclusions and summarizations, outlooks and prospects are discussed as follows, which aim to inspire more innovative progress and accelerate the development of this research field from the lab to the industry.VO 2 -based thermochromic smart windows represent an important technology for increasing indoor comfort and reducing energy consumption in the building.Hence, this hotspot is promising and competitive.Some problems still need to be addressed to further widen their practical applications.
a. Idealizing the optical responses including T lum , ΔT sol and ΔT NIR .To facilitate the commercialization of VO 2 -based radiative cooling coatings, ideally ΔT sol and ΔT NIR should be respectively increased to 57% and 100% with high T lum .This is where researchers had made efforts upon micro/nano-engineering.Further investigations on micro/nano-engineering should be revealed with more complex core-shell micro/nanostructures, higher order optical resonances and combining with other chromogenic materials discussed in Other Chromogenic Materials.b.Continuous and various color changes.Color preference is tightly related to local customs with high versatility.For instance, a colorless coating or coloration with light blue/green is acceptable in China 124 .While gold or violet is more attractive in some areas of Southeastern Asia or Arabia 125 .More sophisticated core-shell or multilayer structure design can help us to achieve this goal.c.Stability and lifetime.Vanadium is multivalent transition element, which reveals +3, +4, +5, and mixed valences.VO 2 (M) tends to be oxidized to  116 .e Phase transitions of perovskites by heating or moisture 7 .Figures reproduced with permission from: (c), K. et al. 116 , American Chemical Society V 2 O 5 or their hydroxides, especially a humid environment.This instability leads to low lifetime in practical applications.Therefore, the antioxidizability and hydrophobicity of VO 2 -based thermochromic films are worthy of our attention.VO 2 -based coreshell micro/nano-structure design might already enhance the antioxidizability to some extent, while affect the luminous transmittance of thermochromic films.Introducing hydrophobic groups on the surface of films to trap water molecules by grafting, etc. chemical modification means 126 and protective film fabrication methods, i.e., electrospinning 127 would be bright solutions.d.Superior fabrication techniques for VO 2 -based thermochromic coatings need to be of large-scale, low-cost, high-quality, high efficiency, and ecofriendly, etc. e. Broadening the multifunctionality of smart windows, e.g., introducing flexibility and foldability, integration of solar cells for electricity generation, integrated display for interactivity, etc. f.Exploiting VO 2 in advanced applications for radiative cooling 128 , warming 129 , and camouflage 130,131 : The potential of VO 2 extends beyond smart windows into the realms of radiative cooling, warming, and camouflage technologies.The thermochromic nature of VO 2 allows for the development of coatings that can dynamically regulate heat exchange with the environment, making them ideal candidates for applications where temperature control and thermal management are critical.For radiative cooling, VO 2 coatings could automatically adjust their thermal emissivity to reflect more solar energy and emit more thermal radiation, thereby reducing the need for artificial cooling systems.In the context of warming, VO 2 could be engineered to absorb more solar energy while reducing thermal radiation loss, providing an energy-efficient approach to heating.Moreover, in the field of camouflage, VO 2 -based coatings could change their thermal signature in response to environmental conditions, making objects less detectable by infrared sensors.The challenge lies in tailoring the phase transition temperature of VO 2 to operate effectively under varying conditions and integrating these coatings into practical systems while maintaining their optical and thermal performance.Future research should focus on the scalable production of VO 2 coatings with tailored properties, their integration with other advanced materials for enhanced performance, and the development of dynamic control systems for responsive thermal management.
To further elucidate the challenges and corresponding solution paths in the development of VO 2 -based thermochromic smart windows, we have prepared a summary, as depicted in Fig. 8.This figure provides readers with a clear visualization of the strategies proposed to address the challenges identified in this review, facilitating a deeper understanding of the research landscape and potential avenues for future exploration.

Fig. 1
Fig. 1 Overview of the VO 2 -based thermochromic coatings for smart windows.a Typical optical properties of VO 2 thin films in visible and NIR regions and corresponding crystal structures: the monoclinic (M) insulating phase at low temperature and rutile (R) metallic phase at high temperature 168 .b Simplified band structures of VO 2 (M) and VO 2 (R) described by molecular orbital diagrams 169 .c The effect of elemental doping on the thermochromism of VO 2 29,40,170 .d Micro/nano-engineering towards performance enhancement of VO 2 -based thermochromic coatings 48,70,171 .e Multifunctional VO 2 -based micro/nano-structures with stability 54 , emissivity 71 , etc. f Controllable fabrication methods of VO 2 -based thermochromic films/coatings 172 .Figures reproduced with permission from: (b), Whittaker et al. 169 , American Chemical Society

Fig. 4
Fig.4 Optical and thermal performance of VO 2 -based films and coatings.a-c Transmittance spectra of a VO 2 and VO 2 @ZnO films at 20 and 80 °C173 , b VO 2 and SiO 2 @TiO 2 @VO 2 coatings at 20 and 100 °C57 , and c VSQ coatings treated at 60 °C and 90% RH for different periods54 .d Absorption spectra of different solutions56 .e, f ΔT sol of VO 2 core-shell NPs-based films, as a function of e shell refractive index and f relative shell thickness53 .g, h ΔT sol of VO 2 inverted core-shell NPs-based films, as a function of g core refractive index and h relative shell thickness, the performance of the plain VO 2 film was shown by the dashed lines.Figures reproduced with permission from: a, Chen et al.58 , American Chemical Society could be further enhanced by integrating low-absorption core materials, like SiO 2 , characterized by a low refractive index and a relative shell thickness exceeding 1.1, as shown in Fig.4g, h.These results reveal that the inversion of the core-shell structure leads to enhancements in Mie scattering effects, suggesting that inverted structures hold greater potential for solar modulation capabilities compared to ordinary structures.

Fig. 5
Fig. 5 Multilayer design to enhance the thermochromism and multifunctionality of VO 2 -based thin films.a Schematic diagrams of a TiO 2 / VO 2 /TiO 2 double-layer AR design 63 .b A RI-tunable ARC/VO 2 bilayer 64 .c A VO 2 /movable ARC bilayer sandwiched between two quartz glass 65 .d A multilayer system as a RCRT window 70 .e Estimated energy-saving map of multilayer VO 2 -based thermochromic smart window.f A tunable Fabry-Pérot cavity based TARC design 71 .g A VO 2 -PDMS-driven radiative cooling coating 77 .Figures reproduced with permission from: (c), Xu et al. 66 , American Chemical Society

Fig. 7
Fig. 7 Schematic illustrations of chromogenic materials undergoing various stimuli.a Liquid-crystal-clad fibers by applying or removing a voltage at 25 °C179 .b The adsorption/desorption of water molecules induced WO 3 gasochromism 180 .c Mechanically triggered establishment of equilibrium between pale yellow DABBF and blue radicals 181 .d Reversible structural transformations of spiropyran under UV/visible light irradiation116 .e Phase transitions of perovskites by heating or moisture7 .Figures reproduced with permission from: (c), K. et al.116 , American Chemical Society

Fig. 8
Fig. 8 Challenges in the development of VO 2 -based thermochromic smart windows
56"-" means data unavailable 64have emerged.For example, Liu et al. utilized a novel RI-tunable (1.47-1.92 at λ = 550 nm) nano sol-gel-based ARC/VO 2 bilayer (Fig.4b) with a ΔT sol of 18.9% and T lum of 44% to enhance the antireflective effect at lower temperatures.This approach aims to maximize ΔT sol for various VO 2 nanosubstrates64.Xu et al. fabricated a VO 2 /Movable ARC bilayer controlled by dual modes using magnetron sputtering, as illustrated in Fig.5c.This approach significantly improved the thermochromic properties, with a ΔT sol of 18.2% and T lum of 42.5%.Moreover, it enabled the two-phase smart regulation of eco-friendly H 2 O through the conversion of liquid and gaseous states.This bilayer system can passively modulate according to the variation in temperature difference between indoor and outdoor environments.

Table 3
Summary of the preparation methods of VO

Table 4
Characteristics of different chromogenic materials